thinner air in the winter

would anyone happen to know if the air is thinner in the fall/winter compared to the summer? When I have been running this past week I have felt like my lungs and legs could not get oxygen they needed. And I had been running all summer and there were no problems. Any ideas would be great. Thanks in advance.
Source: Cystic Fibrosis Exercise and Fitness Forum - Category: Respiratory Medicine Authors: Tags: Exercise & Fitness Source Type: forums